Titanium hydride coated brazing product

ABSTRACT

A one step ceramic brazing material preferably for use in joining ceramics to each other or in joining a ceramic to a metal. The brazing material includes a strip of a eutectic alloy and greater than 3% titanium or titanium hydride in powder form, in a polymer coating thereon. The brazing material incorporates these amounts of titanium or hydride without exhibiting blushing, decreased plasticity, and brittleness in the braze joint. The brazing material is made in the form of a coated strip so that it can be stamped to washers, rings or discs for particular applications.

TECHNICAL FIELD

The present invention relates to new brazing products which may beutilized in a variety of brazing applications, including the joining ofdifferent materials, such as ceramics to metals, as well as the joiningof other materials which are difficult to braze.

BACKGROUND OF THE ART

Ceramic materials are often utilized in applications requiring thejoining of such materials to either other ceramic materials or to metalcomponents. In the electronics industry, vacuum tube production hasutilized such ceramic to metal joints, and currently cesium vapor arclamps utilize brazed ceramic to metal end cap seals. In the automotiveindustry, ceramic is utilized in wear pads which are joined to metalparts to prolong the service life of such parts.

In the past, a two step technique for brazing ceramic materials to metalcomponents was utilized. A molybdenum/manganese paint was first appliedto the ceramic substrate to be joined. The molybdenum/manganese paintedsurface would then be plated with nickel. The metallized surface of theceramic would then be brazed to the desired metal component with asuitable brazing compound. The process was highly time consuming andrequired metallization at extremely high temperatures on the order ofapproximately 1500° C. Metallization of the ceramic material wasrequired due to the inability of brazing compounds of the past tosufficiently wet out onto the ceramic material.

In the 1940's, development of active metal brazing of ceramics began. Inessence, active metal brazing allows a direct brazing of a ceramiccomponent without the need for a prior metallization step. An activecomponent in the brazing filler chemically reacts with the ceramic toform an interfacial compound. The active component in the brazing fillermay be titanium, zirconium, columbium, nobelium, vanadium or chromium.Within the interfacial compound, displaced ceramic cations from theceramic material migrate into the filler. Most commonly, active brazingcompounds are based on binary or ternary metal systems in which at leastone metal is the active metal as discussed above.

U.S. Pat. No. 2,570,248 discloses a mixture of titanium hydride (TiH₂)and a powdered solder, such as a copper solder for in bonding"non-metallic bodies" such as ceramics, porcelains, glasses, carbons,and diamonds, to other non-metallic bodies or to metal components.

The titanium component of this mixture is the active metal componentthat allows direct brazing to the ceramic material. When utilizing aceramic material such as Al₂ O₃, the reaction that takes place betweenan active brazing filler component such as titanium can be representedas:

    3(Ti)+2Al.sub.2 O.sub.3 →3TiO.sub.2 +4(Al)

The aluminum cations are free to move into the braze filler metal as thetitanium oxide in the interfacial compound provides the wetting of theceramic surface which had previously been accomplished with the two stepmetallization process described before.

The process of this patent suffers from certain inherent deficiencies.The coating material includes a mixture of titanium hydride and acopper, silver or gold based solder. The coating is applied by apainting technique wherein irregularities in coating thickness andcoverage often occur. These irregularities lead to localized weakness injoint strength as well as to evaporation of the solder. Furthermore,when such coating materials are utilized to join ceramics in a vacuum,the hydrogen liberated from the titanium hydride must be removedrequiring additional pumping off of such gas.

U.S. Pat. No. 2,857,663 discloses a technique for bonding ceramicmaterials to other ceramic materials or to metals wherein a shim, oftena thin foil, is placed between the ceramic and the component to which itis to be joined. The foil is comprised of at least one metal of thetitanium group (the active metal) and an alloying metal such as copper,nickel, molybdenum, platinum, cobalt, chromium or iron. The alloyingmetal is selected so as to form a eutectic alloy at a temperature belowthe melting temperature of any one of the alloying metals. The titaniumgroup includes metals in group IVb of the standard periodic table, suchas titanium, zirconium, hafnium, and thorium.

The foil technique utilized in this patent allowed a uniform applicationof alloy and active metal between the components to be joined.Additionally, since the hydride form of titanium was not utilized, therewould be no hydrogen gas to remove from the reaction atmosphere.

In an effort to simplify the art of joining ceramic components to otherparts, brazing alloys have been formulated which incorporate up to about2.5% titanium in a eutectic mixture of copper and silver. As discussedabove, it is the titanium group metal that is active in wetting thesurface of the ceramic. There must be provided within the brazing fillera sufficient quantity of titanium to react with the ceramic so as toform a substantial interfacial layer. Yet, in alloys presentlyavailable, there is a functional limit upon the amount of titanium groupactive metal which may be utilized.

Brazing alloys are subject to a phenomenon known as blushing. As abrazing alloy is heated, surface flow of the filler metal may occur. Asthe surface flow of the filler metal increases, depletion of the activemetal component occurs. This causes a depletion in the amount of activemetal available for reaction with the ceramic so as to form theinterfacial compound. A decrease in the interfacial compound results ina weaker bond between ceramic or ceramic/metal components joined by suchbrazing alloys. Brazing materials presently available containingtitanium or titanium in an amount above about 2.5% lead to increasedblushing with the concomitant loss of joint strength.

INCUSIL-ABA® brazing material, a product of the Wesco Division of GTEProducts Corporation, is typical of the brazing alloys presentlyavailable utilizing titanium as an alloy component. The recommendedbrazing temperature of this alloy is from 715°-740° C. After the brazedcomponents are heated to this temperature, they are cooled at acontrolled rate. It is well known that the ceramics have a coefficientof thermal expansion substantially lower than that of metallicmaterials. One of the major difficulties in attaining a brazed joint ofsufficient strength between these materials concerns the high stressgenerated when the ceramic/metal structure cools after brazing. If thisstress is not relieved, or if not redirected so as to strengthen thebond, joint failure will result.

The degree of plasticity a particular brazing alloy provides is one wayin which the stress resulting from joint cooling may be alleviated. Asthe titanium content of a brazing alloy exceeds above about 2-3 weightpercent, a hardening of the brazing alloy occurs so as to limit itscapability of reducing joint structural stress as discussed above.Furthermore, it is often desirable to extrude a brazing alloy so as toform a sheet which may be cut into washers, rings or other shapes inaccordance with joint configuration. The increase of titanium content,as discussed above, will increase alloy brittleness so as to make suchextrusion and cutting procedures highly difficult.

The one step solid alloy brazing materials currently available arelimited, in that such materials may incorporate only about 2 to 3 weightpercent of a titanium group active metal, and this does not provide foroptimum brazed ceramic joints. What is needed, therefore, is a one stepbrazing material that may be extruded into sheets and cut into variousshapes wherein an optimum amount of titanium group active metal isprovided in the material to ensure that a brazed joint of optimumquality is achieved.

SUMMARY OF THE INVENTION

A one step brazing product for joining ceramics to ceramics or metalshas now been formulated which overcomes the deficiencies of the priorart. This brazing product may be extruded into sheets and cut intovarious shapes which conform to the shape of the desired joint.

This brazing product comprises an article of an alloy filler metal of atleast two metals for forming a braze joint when heated to an appropriatebrazing temperature, and a coating upon at least a portion of thearticle. The coating includes an activating agent of a Group IVbtransition metal or transition metal hydride in powder form and a binderfor retaining the transition metal or hydride upon the alloy article.

The Group IV transition metal or hydride is preferably titanium,zirconium or mixtures thereof in powder form. These agents activate manydifferent base materials so that the filler metal alloy will bondthereto. In particular, these components activate ceramics, diamonds,gemstones and other difficult to wet materials. They are also useful foractivating stainless steel or other metals or alloys which form surfaceoxides that would otherwise interfere with filler metal bonding.

Another embodiment of the invention relates to a method for joining afirst material to a second material using the brazing products describedherein by placing the above-described brazing product adjacent orbetween the first and second materials and heating those materials to anappropriate temperature for forming a braze joint. The activating agentfacilitates bonding to the first and second materials by forming aninterfacial compound therewith which enables bonding thereto by themelted filler metal. Thus, the present invention includes a one-stepmethod which greatly simplifies and improves upon prior art techniques.

DETAILED DESCRIPTION OF THE INVENTION

The preferred brazing product of this invention utilizes a strip of aeutectic alloy as the filler metal. A conventional braze alloy which ismost desirable from a melting temperature standpoint is thesilver-copper eutectic composition of 72% silver and 28 copper,optionally containing a small amount of lithium (up to 0.5% by weightbased on the weight of the silver and copper). Preferred alloys includePremabraze 616Braze 716 and Braze 721, each available fromLucas-Milhaupt, Inc., Cudahy, Wisconsin. Other suitable silver-copperalloys include from about 60 to 85 weight percent silver, and from about15 to 40 weight percent copper.

The present invention may alternately include alloys ofsilver-copper-nickel, or silver-copper-indium. The silver-copper-nickelalloys comprise from about 60 to 85 weight percent silver, from 15 to 40weight percent copper and from about 0.2 to 2.5 weight percent nickel,while the silver-copper-indium alloys comprise from about 50 to 70weight percent silver, from about 15 to 35 weight percent copper, andfrom about 10 to 20 weight percent indium. The eutectic alloys of thesemetals are preferred, so that the lowest braze temperatures can beattained.

In addition, a wide range of other conventional filler alloys can beincluded when the invention is to be used for joining similar ordissimilar base materials or components. Alloys containing copper, zinc,nickel, cadmium, tin, silver, gold, molybdenum, cobalt, or palladium,along with additives such as silicon, phosphorus, boron or the like aresuitable. The METGLAS® nickel base materials (products of Allied Corp.,Morristown, New Jersey and gold-nickel alloys (82/18) are specificexamples of additional braze materials, although these have highermelting temperatures than the silver-copper alloys mentioned above.

These alloys can be used as a single strip or layer, or as a "sandwich"(one or two layers adjacent a copper or nickel strip or core). A varietyof "sandwich" products are available from Handy & Harman under thetradename TRIMET. The alloy is generally made in the form of a thinstrip having a width of between 0.02 and 2" (or more if desired) and athickness of 0.001 to 0.25". When thicker strips are desired, thetri-layer "sandwich" construction, wherein two outer layers of thefiller metal alloy are placed about an inner copper or nickel layer, isadvantageous.

The outer layers of this alloy in the form of strip are rolled onto eachside of the layer of copper or nickel. The rolling is completed untilthe thickness of the clad layer is about 0.0025 inch for each of the twoouterlayers and 0.005 inch for the pure copper core. Thus, the overallthickness of this trilayer braze product is approximately 0.01 inches.This product is then cut to conform to the shape of the joint and placedbetween the materials which are to be joined.

Any Group IVb transitional metal or hydride may be used as the activemetal or activating agent of this invention, such as titanium,zirconium, hafnium, thorium or hydrides or mixtures thereof. Of these,titanium or zirconium hydrides or mixtures thereof are most preferred.Due to cost and availability, titanium hydride and titanium metal, inpowder form, are the optimum choices. The particle size of the powder isnot critical and can be routinely selected by one skilled in the art forthe intended joining operation. These agents thus activate the oxides,carbides or nitrides of ceramic materials or on the surface of alloys ormetals so that the filler metal can bond thereto.

The transition metal or hydride in powder form is relatively uniformlydispersed in a binder for application to the alloy strip. As notedabove, the binder is preferably an organic material which, when heatedto the brazing temperature, volatizes without substantially interferingwith or contaminating the braze joint. The most preferred organicmaterials are thermoplastic polymers, such as the acrylic polymers. Itis also possible to utilize as the binder polymethacrylate polymers;elastomers (such as butyl, polyisobutylene or polyisoprene rubber or thelike); polyacrylates; polystyrene; polyethylene oxide polymers;polyacrylonitrile; polyethylene glycols and alkyl (preferably methyl)ethers thereof; polyaliphatic hydrocarbons (such as paraffins, olefins,olefinic copolymers or mixtures thereof); cellulose and its derivatives;and natural gums or resins thereof.

In a preferred embodiment, a silver-copper eutectic alloy strip iscoated with a mixture of titanium hydride powder in a solution of anacrylic polymer. A typical mixture for use in the present inventioncomprises about 50 weight percent titanium hydride and about 50 weightpercent Acryloid F-10 (a 30 to 50% solution of an acrylic polymer in asolvent). The solvent is removed from the acrylic solution by heating sothat a coating of titanium hydride and polymer remains on the strip. Theacrylic polymer binds the titanium hydride securely to the eutecticalloy so that the coated alloy may be stamped and cut into appropriatelyshaped washers and discs conforming to the particular shape of the brazejoint. The acrylic polymer utilized as the binder volatizes attemperatures above about 800° F., leaving little or no residue whichdoes not interfer or contaminate the braze joint.

The present invention includes greater than 3 weight percent, andpreferably greater than about 4 weight percent, and most preferably 6weight percent titanium or more, based on the total weight of alloy andtitanium hydride in the brazing material. As discussed above, ceramicbrazing alloys of the past have been limited to about 2 to 3 weightpercent titanium due to the detrimental effects of greater quantities oftitanium upon the physical properties of the brazing material. As notedabove, alloys utilizing titanium group active metals may onlyincorporate that amount before the blushing, decreased plasticity, andbrittleness of the alloy adversely effects its performance.

The availability of a sufficient amount of transition metal or hydridefor reaction with an oxide or ceramic is directly related to the amountof interfacial compound which may form. It is the formation of theinterfacial compound, as discussed above, that allows the alloyingmetals to adhere to the oxide or ceramic. It has been discovered that ahigher concentration than 2 to 3% of titanium group active metals yieldsbetter quality, higher strength braze joints. Thus, the presentinvention utilizes greater than about 3 weight percent of titanium so asto achieve a more substantial interfacial compound than heretoforepossible from alloys incorporating titanium hydride therein. Byutilizing an acrylic polymer to coat the alloy strip, an increasedweight percentage of titanium hydride is provided so as to allow asubstantially enhanced reaction with the ceramic component. Thisincreased reaction results in increased braze joint strength.

The brazing material of the present invention is also capable of joiningor coating most metals (i.e., iron, copper, nickel and alloys thereofincluding steel, stainless steel, nickel or copper base alloys, etc.)with the exception of hafnium, tantalum, aluminum, titanium, magnesiumor other metals having a melting point which is lower than the brazingtemperature of the filler metal; most oxide ceramics (i.e., alumina,silica, zirconia, etc.); most nitrides; most carbides; diamonds(synthetic or natural); graphite or carbon, and sapphire or likegemstones without the need for prior metallizing procedures.

While it is apparent that the invention herein disclosed is wellcalculated to fulfill the objects above stated, it will be appreciatedthat numerous modifications and embodiments may be devised by thoseskilled in the art, and it is intended that the appended claims coverall such modifications and embodiments as fall within the true spiritand scope of the present invention.

What is claimed is:
 1. A method for brazing a first material to a secondmaterial which comprises:forming an article of a filler metal; coatingat least a portion of the article with a component for activating atleast one of the first and second materials and a binder for retainingsame upon the article; placing the coated article adjacent or betweenthe first and second materials; and heating the first and secondmaterials to an appropriate temperature to form an interfacial compoundof the activating component and at least one of the materials to enablethe article to melt and bond thereto through the interfacial compound,thus forming a braze joint.
 2. The method of claim 1 wherein at leastone of the first or second materials is a ceramic material.
 3. Themethod of claim 2 wherein the ceramic material is an oxide, carbide ornitride surface layer of a metal or alloy material.
 4. The method ofclaim 1 wherein each of the first and second materials comprises aceramic material and which further comprises forming an interfacialcompound with each of the first and second materials.
 5. The method ofclaim 1 which further comprises forming the article in the form of analloy of at least two metals.
 6. A method for brazing a first materialto a second material which comprises:placing a brazing product adjacentto or between a first material and a second material, said brazingproduct comprising (1) an article of a filler metal for forming a brazejoint between said first material and said second material when heatedto an appropriate brazing temperature, and (2) a coating upon at least aportion of the article, said coating comprising a group IVb transitionmetal or transition metal hydride and a binder for retaining thetransition metal or transition metal hydride upon the article when thecoating is applied to said article; and heating the first and secondmaterials to said brazing temperature to enable the article to melt andbond to said materials through an interfacial compound, to thereby joinsaid first and second materials together.
 7. The method of claim 6wherein at least one of the first or second materials is a ceramicmaterial.
 8. The method of claim 6 wherein the ceramic material is anoxide, carbide or nitride surface layer of a metal or alloy material. 9.The method of claim 6 wherein each of the first and second materialscomprises a ceramic material and which further comprises forming aninterfacial compound with each of the first and second materials. 10.The method of claim 6 which further comprises forming the article of analloy of at least two metals.
 11. The method of claim 10 wherein saidalloy is a eutectic alloy of silver and copper.
 12. The method of claim11 wherein the transition metal or transition metal hydride is selectedfrom the group of titanium, zirconium or mixtures thereof.
 13. Themethod of claim 12 wherein the binder includes an acrylic polymer andthe transition metal or hydride is in powder form.
 14. The method ofclaim 13 wherein the transition metal or hydride is present in an amountof at least about 3% by weight of the alloy article.
 15. A method forbrazing a first material to a second material which comprises:placing abrazing product adjacent or between a first material and a secondmaterial, said brazing product comprising (1) an alloy article of afiller material of at least tow metals for forming a braze joint betweensaid first and said second materials when heated to an appropriatebrazing temperature; and (2) a coating upon at least a portion of thearticle which comprises a group IVb transition metal or hydride inpowder form and present in an amount of greater than 3 weight percentbased on the weight of the alloy article, and a binder of an organiccomponent for retaining the transition metal or hydride upon the articlewhen the coating is applied thereto, the transition metal or hydridepowder dispersed substantially uniformly throughout the organic binderand being retained upon the alloy article by the coating, and the bindercapable of volatilizing when heated to the brazing temperature withoutsubstantially interfering with or contaminating the braze joint; andheating the first and second materials to an appropriate temperature toform an interfacial compound of the transition metal or hydride and atleast one of the materials to enable the alloy article to melt and bondto said materials through an interfacial compound, to thereby join saidfirst and second materials together.
 16. The method of claim 15 whereinat least one of the first or second materials comprises a ceramicmaterial.
 17. The method of claim 15 wherein the ceramic material is anoxide, carbide or nitride surface layer of a metal or alloy material.18. The method of claim 15 wherein each of the first and secondmaterials comprises a ceramic material and which further comprisesforming an interfacial compound with each of the first and secondmaterials.
 19. The method of claim 15 which further comprises formingthe alloy article of a eutectic alloy.
 20. The method of claim 19wherein said eutectic alloy is a eutectic alloy of silver and copper.